Escherichia coli is the most common cause of community-acquired urinary tract infection (UTI) and a leading cause of nosocomial UTIs and sepsis. We have focused on a genetic island at argW in E.coli urosepsis strain CFT073 which contains the dsdCXA genes for D- serine utilization and ipuAB, homologs of fimBE, the type 1 fimbriae phase-switch recombinases. A CFT073 dsdA mutant lacking D-serine deaminase is 300-fold more competitive than wild type CFT073 in colonizing the bladder or kidney of experimentally infected mice. Compared to CFT073, 44 and 41 genes were respectively up- and down- regulated in the CFT073 dsdA mutant during murine UTI. Up-regulated genes encoded P and F1C fimbriae, hemolysin, OmpF, a dipeptide transporter DppA, and several genes with unknown functions. CFT073 as well as other uropathogenic E. coli show (+) chemotaxis toward D-serine, but not L-serine as is the case E. coli K-12. ipuA has fimB-like ON-to-OFF and OFF-to-ON fimS switching activity during murine UTI. A CFT073 dsdA::lacZ transcriptional fusion undergoes reversible phase-switching albeit by an unidentified mechanism affected by ipuA and ipuB mutations. ipuA and ipuB also control a fimS- independent reversible phase switch of a set of genes encoding secreted products that includes the hemolysin. Therefore, we generated strong support of our original hypotheses that D-serine represents an important signal for regulation of CFT073 virulence genes and growth in the urinary tract and that the dsdCXA-linked recombinases mediate phase-state regulation of important urovirulence and fitness factors besides the type 1 fimbriae. We will investigate the mechanism whereby elevated intracellular D-serine leads to up-regulation of urovirulence and fitness genes and characterize loci aside from fimS that are controlled by the recombinases. We will use the CFT073 dsdA mutant as a model of the DsdA OFF state to study the roles of P fimbriae and hemolysin in colonization of the murine bladder and kidney. The objective of our project is to identify and characterize virulence genes for E. coli involved in serious human diseases. This work will aid development of new vaccines and antimicrobials. To that end, we have recently acquired evidence that the D-serine deaminase can serve as a novel drug target for identification of new antimicrobials.